1. Field of the Invention
The present invention relates to a packet communications system, and more particularly to a communications system for sending and receiving image information and other information in real time over a high-speed packet network synchronizing between a transmitter side and a receiver side.
2. Description of the Prior Art
In recent years, development of communications systems using optical fibers and other high speed transmission paths has greatly increased communication capacity. This has spurred the development of digital communications systems, which are now used not only for digital computer data but also for transmitting digitized image signals, audio signals, and related additional information.
Asynchronous transfer mode (ATM) systems, for example, can operate at a speed in excess of 155 megabits/second, and practical ATM systems are now available for communicating digital image data in real time.
Formal standards for ATM systems have been adopted by the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) and the ATM Forum, for example, and numerous related publications have been issued.
Such image communications systems employing high-speed packet networks are disclosed in, for example, Japanese Patent Laid-open Publication 5-14763, in gazette, as a first reference, Japanese Patent Laid-open Publication 5-14399, in gazette, as a second reference, and U.S. Pat. No. 5,386,436, as a third reference.
According to the first reference, clocks at transmitter and receiver terminals are synchronized in subordination to a network clock of a high-speed packet network. On the transmitter terminal, image data are transmitted in conjunction with transmission frame synchronizing signals that are generated based on the subordinately synchronized clock.
On the receiver terminal, meanwhile, reproduction-side synchronization is effected on the frames using both reception frame synchronizing signals generated on the basis of a subordinately synchronized clock and transmission-reproduction frame synchronizing signals generated on the basis of timing packets added to the transmission image packets.
In the disclosure of the second reference, synchronization packets added to transmitted image signals are detected on the receiver terminal, as in the first reference, and image frame synchronizing signals and image clocks are generated from moving averages of arrival times of the synchronizing information based on the synchronization packets, and the synchronization of image frames is thereby achieved.
The third reference provides a synchronization method using statistical characteristics of adaptive differential PCM (ADPCM) signals constituting transmission data.
The configurations described in the forgoing conventional systems, however, are subject to following problems.
As concerning the first reference:
(1) The transmission-reproduction frame synchronization signal (FST1) generated by a timing control means is generated based on a timing packet (PT), and therefore, when a PT cannot be detected because of an error such as a cell loss or packet destruction on the ATM network occurring on the packet communications network, the frame synchronization signal FST1 cannot be generated normally. Thus, image data cannot be stored in a frame memory unit, which leads to the breakdown of the image.
(2) Also, intervals between timing packet receptions are not constant due to the influence of fluctuations on the ATM networks, etc., and therefore, there may be a problematic case where synchronization cannot be achieved because of the effects of network characteristics.
(3) Moreover, the system cited in the first reference will not function without the timing control means, resulting in very large circuit sizes.
As concerning the second reference:
(4) Image frame synchronizing signals are generated using an arrival time calculation means, moving average calculation means, observation window setting means, and frame synchronizing signal generation means, but this is all dependent on the synchronization packet Ps. Accordingly, although some degree of resilience to both packet destruction and fluctuations is effected by taking of moving averages, in the event that synchronization packets Ps are continually destroyed, for example, and if a limiting value thereof is exceeded, the images will break down.
(5) Moreover, the arrival time calculation means, moving average calculation means, observation window setting means, and frame synchronizing signal generation means are circuits involved in computations and thus result in a system having a very large circuit size.
(6) According to both the first and the second references, moreover, there is only one timing packet Pt or one synchronization packet Ps in one frame period, so that, when these are destroyed, it becomes impossible to receive any of the image data in the frame period, resulting in that the error is compounded.
The fundamental problem with the conventional systems disclosed in the first and the second references resides in the fact that the synchronization signals are generated on the receiver side based on unstable synchronization packets which have been detected on the receiver side.
In the conventional system disclosed in the third reference, unlike the art cited in the first and second references, there is no dependency on unstable synchronization packets, but there is a problem nevertheless in that this system only applies to a case where the data being communicated are adaptive differential PCM (ADPCM) signals.
The present invention has been developed to solve the problems inherent to the conventional systems, and an essential object of the present invention is to provide an improvement of an image packet communications system.
In the first aspect of the present invention, provided is an image packet communications system for communicating image data information in real time between a transmitter side and a receiver side through a network, wherein the transmitter comprises:
a transmission clock generator for generating a transmission clock subordinately synchronized to a given reference clock; and
a packet generator for generating image packets from input image signals based on the transmission clock, meanwhile, the receiver comprises:
a reception clock generator for generating a reception clock subordinately synchronized to the given reference clock, so that the reception clock is synchronized with the transmission clock;
a packet receiver receiving the image packets with a network delay to be stored in a buffer memory group; and
a buffer controller for generating a reception standard signal as a control signal for controlling the writing and reading of the image packets to and from the buffer memory group, based on the reception clock.
In this arrangement, the transmitter further comprises a transmission standard signal generator for generating a transmission standard signal based on the transmission clock and the packet generator generates the image packets with image packet headers attached thereto to produce transmission packets, with reference to the transmission standard signal, and wherein the packet receiver is comprised of a separator means for separating the image packets and the image packet headers from each other so that the buffer controller generates an address signal based on information of the image packet headers supplied from the packet receiver.
In the second aspect of the present invention, the buffer controller is supplied with a reference signal (REF) from the outside as a frame standard signal, and when detecting that a time differential between the reception standard signal generated based on the reception clock and the reference signal (REF) input from the outside exceeds a predetermined value, the writing and reading of the image packets to and from the buffer memory group is so controlled such that the image data output from the buffer memory group has a suitable volume for the reference signal.
In the third aspect of the present invention, the transmitter further comprises a buffer memory group for storing the input image signals and a buffer controller for controlling the writing and reading of the image signals to and from the buffer memory group, and wherein the buffer controller is supplied with a reference signal (REF) from the outside as a frame standard signal, and when detecting that a time differential between the transmission standard signal generated by the transmission standard signal generator and the reference signal (REF) input from the outside exceeds a predetermined value, the writing and reading of the input image signals to and from the buffer memory group is so controlled such. that the image data output from the buffer memory group has a suitable volume for the reference signal.
In the fourth aspect of the present invention, the network is comprised of one or more packet switching devices, each packet switching device including:
a plurality of transceivers for transmitting and receiving image data packets; and
a switch standard signal generator for selecting any transceiver from among a plurality of the transceivers, and generating switch standard signals from the image packet signals received by the transceiver, so that the switch standard signals are subordinately synchronized to the transmission standard signal generated by the transmission standard signal generator, thereby subordinately synchronizing any of the selected transceiver to the transmission standard signal, whereby the transmission packets are transmitted to the network with the timing subordinately synchronized to the transmission standard signal.
In the fifth aspect of the present invention, the packet generator completes the transmissions of the image packets and packet headers belonging within a period of said transmission standard signal, within a time period obtained by subtracting the maximum network fluctuation from said transmission standard signal.
According to the first aspect of the present invention, transmissions are conducted with a clock subordinately synchronized to the network clock and frame signals generated based on the clock established as the synchronization standard at the transmitter terminal, and in the meanwhile, transmissions are conducted with a clock subordinately synchronized to the network clock and frame signals generated based on the clock established as the synchronization standard at the receiver end, thus affording the benefit of being able to build a communications system where the synchronization between the transmitter end and the receiver end can be guaranteed.
Furthermore, the image packets are transmitted after attaching image packet headers thereto, wherefore, even when a packet loss or error occurs in the network, and a transmission packet is lost, that transmission packet error has no effect on the other transmission packets, and normally received transmission packets are securely received and output. Accordingly, an effect is afforded in that high image quality can be maintained. in the transmissions even when packet losses or errors occur.
According to the second aspect of the present invention, when the synchronization period of the network clock in the high-speed packet network differs from the period of the reference signal at the receiver side, it is possible to adjust the image data volume so that the image data received from the network are made suitable to the reference signal period. Accordingly, it is possible to build systems wherein equipment synchronized to the network clock is connected to equipment synchronized to the reference signal, thereby affording the effect of being able to guarantee frame synchronization for all equipment in the system.
According to the third aspect of the present invention, when the synchronization period of the network clock in the high-speed packet network differs from the period of the reference signal, the volume of image data can be adjusted so that the image data input synchronized to the reference signal are made suitable to the period of the network clock. This makes it possible to interconnect equipment synchronized to the reference signal and equipment synchronized to the network clock, thus affording the effect of being able to guarantee the frame synchronization of all equipment in the system. The image data volume is adjusted in frame units, so that high image quality can be preserved.
According to the fourth aspect of the present invention, it is possible to build an image packet communications system assuring the frame synchronization of all equipment included in the transmitter, the high-speed packet network, and the receiver, thereby affording the effect of maintaining high image quality.
According to the fifth aspect of the present invention, the image data in each frame are transmitted without fail within the frame period, even when fluctuations occur in the high-speed packet network, thereby affording the effect of realizing an image packet communications system assuring high image quality.
The first, second, third, fourth, and fifth aspects of the present invention all afford the benefit of being realizable in a simple configuration. Furthermore, the present invention can be implemented without detecting any kind of signal from the data, thus providing the benefit of being applicable irrespective of the data being communicated.